EP 1115943 (=U.S. Pat. No. 6,468,390) discloses a method for cooking wherein early cooking liquor with a high content of dissolved hemicellulose, mainly in the form of Xylan, is withdrawn from the cooking process, most suitably directly after the end of the impregnation step, and is then reintroduced in the end of the cooking process where the hemicellulose is allowed to precipitate on the softened wood chips by giving it an extended retention time. With this method it is possible to increase the yield by at least 1% and also improve the beatability of the pulp. EP 1115943 discloses in detail the process for selective precipitation of hemicellulose/Xylan and prior art at the date of filing.
U.S. Pat. No. 3,354,029 discloses a process whereby cooking liquor is added to the last 15 minutes of the cooking process and alkalinity is reduced by adding an acid to reduce pH to 11.5-13, so that a precipitation of lignin, so called lignin condensation, and to some extent hemicellulose occurs. The process in EP 1115943 is different from this process in that the object is to precipitate Xylan, and not obtain a lignin condensation at all, since high alkalinity must be maintained in the process.
U.S. Pat. No. 3,937,647 discloses another process where it is only desired to precipitate organic material, i.e. also lignin, where this precipitation process is activated by lowering pH to a value under 11, specifically a pH in the range of 5.5-10.
U.S. Pat. No. 3,802,956 discloses another such process with precipitation in the end of the cooking process with a short retention time allowed for the dissolved organic material together with the softened wood chips.
Upon application of the method according to EP 1115943 it was found that the amount of dissolved hemicellulose during the cooking process varies with time, and that the characteristics of the hemicellulose that has just been dissolved also vary with time. The hemicellulose that has dissolved early in the cooking process begins to degrade, i.e. the hemicellulose in the form of in the Xylan chains is broken down, whereby the yield increasing effect of the Xylan precipitation from the early dissolved Xylan is completely or partly lost. During the cooking process Xylan dissolves easier from the surface of the partly softened wood chips, which is why the likewise early dissolved hemicellulose on average contains longer chains than the hemicellulose which is dissolved later from the interior of the softened wood chips. Among others, Herbert Sixta's “Handbook of Pulp”, Vol. 1, discloses the rate of dissolution for different kinds of hemicellulose. Xylan is dissolved relatively quickly and after 100 minutes in an alkali cooking process, with a maintained temperature of 170° C., 25% of Xylan has been dissolved after 100 minutes, while 25% of Glucomannan has been dissolved after 200 minutes. Hexenuronic acids are dissolved even more quickly. Already after less than 100 minutes more than 75% has been dissolved from the wood. It is often desirable to remove hexenuronic acids from the pulp because pulp with a high content of Hexenuronic acids is hard to bleach. It is often necessary to use very aggressive bleaching steps with either a high temperature or highly effective bleaching chemicals.
The late dissolved hemicellulose has also been subjected to the high cooking temperature for a longer period of time, a factor that also affects this late dissolved hemicellulose.
To be able to fully optimize a so called Xylan cooking process according to EP 1115943 it has been found suitable to withdraw cooking liquor with the different fractions of early and late dissolved hemicellulose, before these cooking liquors are reintroduced in the end of the cooking process where a selective precipitation of hemicellulose occurs on the softened wood chips, i.e. without any significant lignin precipitation.
These withdrawals of cooking liquor are most suitably performed in combination with the addition of replacement fluid, so that the amount of hemicellulose in the cooking liquor may be diluted and facilitate the dissolving the hemicellulose still bound in the wood chips.
According to the present invention, a first withdrawal is initiated during the cooking process at a position between the later half of the impregnation step and the first quarter of the cooking zone (measured in time), and a second withdrawal is performed later in the cooking zone, where the liquids that have been withdrawn with their contents of dissolved hemicellulose are reintroduced to the last phase of the cooking zone to there be able to precipitate on the wood chips softened in the cooking process. This way it is possible to quickly withdraw the early dissolved hemicellulose, which has a long chain length, and continue to dissolve more hemicellulose in a later phase, whereupon these liquids with the “freshly” dissolved hemicellulose are reintroduced to the last phase of the digester for precipitation on wood chips, when the cleavage of carboxylic acid groups in the Xylan start to reduce the solubility of the Xylan. By withdrawing the primary dissolved hemicellulose early it is possible to prevent that the degradation of Xylan reaches a point where the yield increasing effect is lost.
The invention may be applied on both steam phase digesters and hydraulic digesters, in both single-vessel and two-vessel cooking systems.